A Confrontation with a Rival
SPOILERS AHEAD! BE WARNED!
Who is the greatest threat? Or alternatively, who is weakest?
Game Instructions: Successfully confronting a Rival will usually remove them, and reduce Rivals' Progress enough to let you continue your Expedition. If you remove all your named Rivals, you'll only have to deal with the general rabble of second-raters, who are less of a threat.
Unlocked with a redirect from Confront a Rival